In the manufacture of semiconductor devices and other products, ion implantation is used to dope semiconductor wafers, display panels, or other workpieces with impurities. Ion implanters or ion implantation systems treat a workpiece with an ion beam, to produce n or p-type doped regions or to form passivation layers in the workpiece. When used for doping semiconductors, the ion implantation system injects a selected ion species to produce the desired extrinsic material, wherein implanting ions generated from source materials such as antimony, arsenic or phosphorus results in n-type extrinsic material wafers, and implanting materials such as boron, gallium or indium creates p-type extrinsic material portions in a semiconductor wafer.
FIG. 1A illustrates an exemplary ion implantation system 10 having a terminal 12, a beamline assembly 14, and an end station 16. The terminal 12 includes an ion source 20 powered by a high voltage power supply 22 that produces and directs an ion beam 24 to the beamline assembly 14. The beamline assembly 14 has a beamguide 32 and a mass analyzer 26 in which a dipole magnetic field is established to pass only ions of appropriate charge-to-mass ratio through a resolving aperture 34 at an exit end of the beamguide 32 to a workpiece 30 (e.g., a semiconductor wafer, display panel, etc.) in the end station 16. The ion source 20 generates charged ions that are extracted from the source 20 and formed into the ion beam 24, which is directed along a beam path in the beamline assembly 14 to the end station 16. The ion implantation system 10 may include beam forming and shaping structures extending between the ion source 20 and the end station 16, which maintain the ion beam 24 and bound an elongated interior cavity or passageway through which the beam 24 is transported to the workpiece 30 supported in the end station 16. The ion beam transport passageway is typically evacuated to reduce the probability of ions being deflected from the beam path through collisions with air molecules.
Low energy implanters are typically designed to provide ion beams of a few thousand electron volts (keV) up to around 80–100 keV, whereas high energy implanters can employ linear acceleration (linac) apparatus (not shown) between the mass analyzer 26 and the end station 16 to accelerate the mass analyzed beam 24 to higher energies, typically several hundred keV, wherein DC acceleration is also possible. High energy ion implantation is commonly employed for deeper implants in the workpiece 30. Conversely, high current, low energy ion beams 24 are typically employed for high dose, shallow depth ion implantation, in which case the lower energy of the ions commonly causes difficulties in maintaining convergence of the ion beam 24.
In the manufacture of integrated circuit devices, display panels, and other products, it is desirable to uniformly implant the dopant species across the entire surface of the workpiece 30. Different forms of end stations 16 are found in conventional implanters. “Batch” type end stations can simultaneously support multiple workpieces 30 on a rotating support structure, wherein the workpieces 30 are rotated through the path of the ion beam until all the workpieces 30 are completely implanted. A “serial” type end station, on the other hand, supports a single workpiece 30 along the beam path for implantation, wherein multiple workpieces 30 are implanted one at a time in serial fashion, with each workpiece 30 being completely implanted before implantation of the next workpiece 30 begins.
The implantation system 10 includes a serial end station 16, wherein the beamline assembly 14 includes a beam scanner 36 that receives the ion beam 24 having a relatively narrow profile (e.g., a “pencil” beam), and scans the beam 24 back and forth in the X direction to spread the beam 24 out into an elongated “ribbon” profile, having an effective X direction width that is at least as wide as the workpiece 30. The ribbon beam 24 is then passed through a parallelizer 38 that directs the ribbon beam toward the workpiece 30 generally parallel to the Z direction (e.g., generally perpendicular to the workpiece surface).
Referring also to FIGS. 1B–1J, the beam scanner 36 is further illustrated in FIG. 1B, having a pair of scan plates or electrodes 36a and 36b on either lateral side of the beam path, and a voltage source 50 that provides alternating voltages to the electrodes 36a and 36b, as illustrated in a waveform diagram 60 in FIG. 1C. The time-varying voltage potential between the scan electrodes 36a and 36b creates a time varying electric field across the beam path therebetween, by which the beam 24 is bent or deflected (e.g., scanned) along a scan direction (e.g., the X direction in FIGS. 1A, 1B, and 1F–1J). When the scanner electric field is in the direction from the electrode 36a to the electrode 36b (e.g., the potential of electrode 36a is more positive than the potential of electrode 36b, such as at times “a” and “c” in FIG. 1C), the positively charged ions of the beam 24 are subjected to a lateral force in the negative X direction (e.g., toward the electrode 36b). When the electrodes 36a and 36b are at the same potential (e.g., zero electric field in the scanner 36, such as at time “e” in FIG. 1C), the beam 24 passes through the scanner 36 unmodified. When the field is in the direction from the electrode 36b to the electrode 36a (e.g., times “g” and “i” in FIG. 1C), the positively charged ions of the beam 24 are subjected to a lateral force in the positive X direction (e.g., toward the electrode 36a).
FIG. 1B shows the scanned beam 24 deflection as it passes through the scanner 36 at several exemplary discrete points in time during scanning prior to entering the parallelizer 38 and FIG. 1D illustrates the scanned and parallelized beam 24 impacting the workpiece 30 at the corresponding times indicated in FIG. 1C. The scanned and parallelized ion beam 24a in FIG. 1D corresponds to the applied electrode voltages at the time “a” in FIG. 1C, and subsequently, the beam 24b–24i is illustrated in FIG. 1D for scan voltages at corresponding times “c”, “e”, “g”, and “i” of FIG. 1C for a single generally horizontal scan across the workpiece 30 in the X direction. FIG. 1E illustrates a simplified scanning of the beam 24 across the workpiece 30, wherein mechanical actuation (not shown) translates the workpiece 30 in the positive Y (slow scan) direction during X (fast scan) direction scanning by the scanner 36, whereby the beam 24 is imparted on the entire exposed surface of the workpiece 30.
Prior to entering the scanner 36, the ion beam 24 typically has a width and height profile of non-zero X and Y dimensions, respectively, wherein one or both of the X and Y dimensions of the beam typically vary during transport due to space charge and other effects. For example, as the beam 24 is transported along the beam path toward the workpiece 30, the beam 24 encounters various electric and/or magnetic fields and devices that may alter the beam width and/or height or the ratio thereof. In addition, space charge effects, including mutual repulsion of positively charged beam ions, tend to diverge the beam (e.g., increased X and Y dimensions), absent countermeasures.
Also, the geometry and operating voltages of the scanner 36 provide certain focusing properties with respect to the beam 24 that is actually provided to the workpiece 30. Thus, even assuming a perfectly symmetrical beam 24 (e.g., a pencil beam) entering the scanner 36, the bending of the beam 24 by the scanner 36 changes the beam focusing, wherein the incident beam typically is focused more at the lateral edges in the X direction (e.g., 24a and 24i in FIG. 1D), and will be focused less (e.g., wider or more divergent) in the X dimension for points between the lateral edges (e.g., 24c, 24e, and 24g in FIG. 1D).
FIGS. 1F–1J illustrate the incident beam 24 corresponding to the scanned instances 24a, 24c, 24e, 24g, and 24i, respectively. As the beam 24 is scanned across the wafer 30 in the X direction, the X direction focusing of the scanner 36 varies, leading to increased lateral defocusing of the incident beam 24 as it moves toward the center, and then improved focusing as the beam 24 again reaches the other lateral edge. For no scanning, the beam 24e proceeds directly to the center of the workpiece 30, at which the incident beam 24e has an X direction width WC, as shown in FIG. 1H. As the beam 24 is scanned laterally in either direction away from the center, however, the time varying focusing properties of the scanner 36 lead to stronger and stronger lateral focusing of the incident beam. For instance, at the outermost edges of the workpiece 30, the incident beam 24a in FIG. 1F has a first left side width WL1, and on the right side, the incident beam 24i in FIG. 1J has a first right side width WR1. FIGS. 1G and 1I illustrate two intermediate beams 24c and 24g having incident beam widths WL2 and WR2, respectively, showing X direction focal variation between the edges and the center of the workpiece 30.
In general, it is desirable to provide uniform implantation of the surface of the workpiece 30, regardless of the particular focal properties of the beam transport and scanning system. Accordingly, conventional systems often undergo a calibration operation to adjust the voltage waveform of the beam scanner 36 to counteract the focal variation of the beam 24 along the scan direction and/or to compensate for other beam irregularities. This is typically done in a point-to-point fashion by measuring a current density profile in a region at or near the workpiece location that results from a beam set to the region. The profile region and the scanner voltage range are subdivided into corresponding intervals. For a given scanner voltage interval, a measurement sensor is located at the position corresponding to the center of the interval, and the beam is directed at the region being measured. Such measurements are then repeated for each of the voltage intervals, and the final scan waveform is adjusted to compensate for profile non-uniformities.
Although the conventional point-to-point scanner calibration techniques may be adequate where the width of the ion beam 24 is narrow and the beam width is relatively constant across the target area, these techniques are less suitable in the case of wider beams 24 and/or in situations where the beam width varies along the scan direction, as in the example of FIGS. 1F–1J. In particular, if the beam 24 is wide and/or variable across the target area, the point-to-point technique fails to account for the workpiece dose produced by the beam some distance from the beam center. This situation is particularly problematic with low energy ion beams 24 that experience space charge expansion (e.g., lateral divergence in the scan or X direction).
Another consideration is the amount of beam overscan, which includes the extent to which the ion beam 24 is scanned past the edges of the workpiece 30, as illustrated in FIG. 1E. In most applications, the beam 24 must be scanned beyond the target by an amount related to the width of the beam 24 in order to achieve uniform implantation of the entire workpiece surface. However, the time that the scanned beam 24 spends outside the target area is essentially wasted, and detracts from the system scan efficiency, defined as the time spent on the target workpiece 30 divided by the total scan time.
Accordingly there is a need for improved ion beam scanner calibration techniques by which uniform implantation can be facilitated, and which facilitates improved scan efficiency by determining the minimum overscan required to achieve uniform implantation of a workpiece.